Merge staging into closure-size

The most complex problems were from dealing with switches reverted in
the meantime (gcc5, gmp6, ncurses6).
It's likely that darwin is (still) broken nontrivially.

pkgs/development/libraries/gmp/6.x.nix

@@ -3,11 +3,11 @@
 with { inherit (stdenv.lib) optional optionalString; };
 
 let self = stdenv.mkDerivation rec {
-  name = "gmp-6.0.0a";
+  name = "gmp-6.1.0";
 
   src = fetchurl { # we need to use bz2, others aren't in bootstrapping stdenv
     urls = [ "mirror://gnu/gmp/${name}.tar.bz2" "ftp://ftp.gmplib.org/pub/${name}/${name}.tar.bz2" ];
-    sha256 = "1bwsfmf0vrx3rwl4xmi5jhhy3v1qx1xj0m7p9hb0fvcw9f09m3kz";
+    sha256 = "1s3kddydvngqrpc6i1vbz39raya2jdcl042wi0ksbszgjjllk129";
   };
 
   #outputs TODO: split $cxx due to libstdc++ dependency; maybe port to gmp5;